The present invention relates to a method of compensating for misregistration during operation of a printing press.
In achieving high printing quality on a printing unit, the positional accuracy of the color separations relative to one another is of paramount importance. This positional accuracy is defined by the register, i.e., by any misregistration. The register should be kept as constant as possible, which means in other words that misregistration should be avoided. Misalignment can be critical after a pause in printing and in particular after washing of the rubber blanket. It is particularly critical where the rubber blanket is washed frequently, for example when printing fluffing papers and in the case of relatively long critical print runs. The misalignment after a pause in printing may be several hundredths of a millimeter, depending on the type of press and the number of printing units. When printing is re-started, it can take up to several hundred sections until the misalignment has returned to the original steady-state value before the pause. It thus takes a very long time before the press operator reaches a good section. Misalignment is primarily caused by changes in the characteristics of the departure of the paper from the rubber blanket.
A method is known in an automatic registration measurement and control device from the Heidelberger Druckmaschinen company of Heidelberg, Germany, having the designation Autoregister CPC 42. In this device, registration marks are printed in each printing unit at the edges of the paper sheet, indicating the position of the color separation on the sheet. Making use of these printed registration marks, the misregistration of the individual colors respective to one another are determined and positioning commands are calculated therefrom in order to make the necessary register corrections. The corrections are then applied, thus compensating for the misregistration.
A further improved method of regulating the register in a sheet-fed rotary printing press when the print speed changes is known from German Patent Application No. 101 32 266 A1.
However, both compensation methods are complex in terms of regulation and hardware, and in addition the register shift after a pause in printing can only be corrected once the measurement system has determined the shift and passed the information to the control system.
Furthermore, it is generally conventional practice to correct misregistration by manual adjustment. However, this must be repeated after every pause in printing and in particular after every washing operation; there is no provision for automation.